Exclude Firefox 7 & 8 from performance reporting

The Navigation Timing API is broken in Firefox 7 and 8 and reports
inaccurate measurements. See <https://bugzilla.mozilla.org/691547>. The
sources I've consulted (e.g. <http://goo.gl/wrTCG>) recommend ignoring
measurements from these browsers. This change adds a check of
navigator.userAgent to prevent FF 7 & 8 from reporting measurements.

Change-Id: I769a92ea211da488eac97fb89bffa09a00d74329
Merge "Exclude Firefox 7 & 8 from performance reporting"